Conventionally, an electromagnetic induction type contactless power transmission device includes a metal detection device. When a metal is placed on a setting surface, the metal detection device prevents the metal from being inductively heated when supplying power to an electric instrument.
For example, in patent document 1, a metal detection device is a frequency following circuit that searches for a resonance frequency to determine whether (1) nothing is placed on the setting surface, (2) a power receiving circuit is correctly placed on the setting surface, and (3) a foreign object of metal or the like is placed on the setting surface.
In patent document 2, a metal detection device monitors changes in the amplitude of a coil voltage caused by load modulation to detect a foreign object of metal or the like.
Furthermore, in patent document 3, a metal detection device changes a signal applied to a primary coil between two intensities and monitors the power consumed at the primary side to check whether a foreign object exists on the setting surface.